EN 1993-1-8 Connection Design — Bolts, Welds, Fin Plates & End Plates

Complete reference for EN 1993-1-8:2005 + A1:2014 steel connection design under Eurocode 3. Covers bolt shear, bearing, and tension resistance (Table 3.4), fillet weld directional and simplified methods (Clause 4.5.3), block tearing (Clause 3.10.2), tying resistance for structural integrity (Clause 6.2), fin plate connection design (SN017), and extended end plate design (SN041). Includes a worked bolt group example with combined shear and moment using the elastic method.

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EN 1993-1-8 Connection Design — Overview

EN 1993-1-8 governs the design of joints in steel structures under Eurocode 3. It covers bolted connections, welded connections, and the classification of joints by stiffness (rigid, semi-rigid, nominally pinned) and strength (full-strength, partial-strength, nominally pinned). The UK National Annex provides the nationally determined parameters (NDPs) for the UK market.

The governing partial factors for connection design per EN 1993-1-8 and the UK NA are:

EN 1993-1-8 gamma_M2 = 1.25 is more conservative than AISC 360 phi = 0.75 (equivalent gamma_M ~ 1.33 vs 1.25, but the AISC phi applies to nominal strength while EN gamma_M2 applies to the characteristic resistance — the net effect depends on the material partial factor conventions). When comparing directly at the connection level, a Grade 8.8 M20 bolt in single shear: Fv,Rd = 94.1 kN (EN 1993-1-8) vs phi-Rn = 76.9 kN (AISC 360 threads included) — the EN value is higher because of the higher fub = 800 MPa vs A325 fu = 725 MPa, not because of the partial factor.

Bolt Categories per EN 1993-1-8 Table 3.1

EN 1993-1-8 classifies bolted connections into five categories based on the loading type and whether slip resistance is required:

Category A is the default for shear connections in building structures. Categories B and C require controlled preloading per EN 1090-2. For most UK building connections (fin plates, end plates, angle cleats), Category A bearing-type bolts are adequate.

Bolt Shear Resistance (EN 1993-1-8 Table 3.4)

The design shear resistance per shear plane for Category A bolts:

Fv,Rd = alpha_v * fub * A / gamma_M2

Where:

• alpha_v = 0.6 for grades 4.6, 5.6, 8.8 (shear plane passes through threaded portion)
• alpha_v = 0.5 for grade 10.9 (shear plane through threaded portion)
• alpha_v = 0.6 for all grades if shear plane passes through unthreaded shank
• A = As (tensile stress area) when threads in shear plane; A = gross shank area when unthreaded
• gamma_M2 = 1.25

Bolt Shear Capacities — Metric Coarse Thread (EN ISO 4016/4017)

For double shear connections, multiply Fv,Rd by 2.0.

Bolt Bearing Resistance (EN 1993-1-8 Table 3.4)

The design bearing resistance per bolt:

Fb,Rd = k1 * alpha_b * fu * d * t / gamma_M2

Where:

• alpha_b = min(e1 / 3d0, p1 / 3d0 - 1/4, fub / fu, 1.0)
• k1 = min(2.8 _ e2 / d0 - 1.7, 1.4 _ p2 / d0 - 1.7, 2.5) for edge bolts
• k1 = min(1.4 * p2 / d0 - 1.7, 2.5) for inner bolts
• d = bolt diameter (mm), t = plate thickness (mm), d0 = hole diameter (mm)
• fu = ultimate tensile strength of the connected plate material
• gamma_M2 = 1.25

Bearing Capacities — M20 Bolt in S355 Plate (fu = 470 MPa)

For typical UK practice, fin plates of 10-12 mm and end plates of 15-20 mm in S355 provide adequate bearing resistance for M20 Grade 8.8 bolts. Bearing will usually not govern the connection design.

Block Tearing — EN 1993-1-8 Clause 3.10.2

Block tearing is the limit state where a group of bolt holes causes a block of material to tear out. The design resistance for a symmetric bolt group subjected to concentric loading:

Veff,1,Rd = fu * Ant / gamma_M2 + (1/sqrt(3)) * fy * Anv / gamma_M0

An alternative expression (often more critical):

Veff,2,Rd = 0.5 * fu * Ant / gamma_M2 + (1/sqrt(3)) * fy * Anv / gamma_M0

Where:

• Ant = net area subjected to tension
• Anv = net area subjected to shear
• gamma_M0 = 1.00, gamma_M2 = 1.25

For a fin plate with 3 M20 bolts (d0 = 22 mm), plate depth 200 mm, thickness 10 mm, S355:

• Ant = (40 - 22/2) * 10 = 290 mm^2 (top bolt tension area)
• Anv = (200 - 40 - 2.5 _ 22) _ 10 = 1,050 mm^2
• Veff,1,Rd = 470 _ 290 / 1.25 + (1/1.732) _ 355 * 1,050 / 1.00 = 109.0 + 215.2 = 324.2 kN
• Veff,2,Rd = 0.5 _ 470 _ 290 / 1.25 + 215.2 = 54.5 + 215.2 = 269.7 kN (governs)

Design block tearing resistance: 269.7 kN. If V_Ed = 200 kN, utilisation = 0.74 — OK.

Tying Resistance — EN 1993-1-8 Clause 6.2

EN 1991-1-7 requires structural integrity (robustness) for buildings in Consequence Class 2b and above (UK Building Regulations Approved Document A). Horizontal ties must be designed to resist a notional tying force. EN 1993-1-8 Annex A provides the detailed design method.

For a fin plate connection, the tying resistance N_Rd,u is:

For bolts in shear:      N_Rd,u,1 = n * Fv,Rd
For bolts in bearing:    N_Rd,u,2 = n * Fb,Rd
For net section failure: N_Rd,u,3 = 0.9 * Anet * fu / gamma_Mu
                                         (gamma_Mu = 1.10 per UK NA)
For block tearing:       N_Rd,u,4 = Veff,Rd (block tearing)
For fin plate gross:     N_Rd,u,5 = b * t * fy / gamma_M0
For beam web (supported): N_Rd,u,6 = tw * fy * (n1-1)*p1 / gamma_M0

The tying resistance is the minimum of all failure modes. UK practice per the SCI/BCSA Green Books uses a gamma_Mu = 1.10 for the net section tie force check (more lenient than gamma_M2 = 1.25, recognising that material over-strength assists tie capacity).

For a 3-bolt M20 fin plate, S355, 200 x 10 mm:

• N*Rd,u,1 = 3 * 94.1 _ 2 (double shear typically) = 564.6 kN
• NRd,u,3 = 0.9 * (200 - 322) * 10 _ 470 / 1.10 = 0.9 _ 1,340 _ 470 / 1.10 = 515.2 kN
• Tie capacity = min(564.6, 515.2, ...) ~ 515 kN

The SCI P358 Green Book provides pre-calculated tying capacities for standardised connections.

Fin Plate Connections — SCI P358 / SN017

The fin plate (shear tab) is the most common simple connection in UK steel construction. Per SCI P358 and EN 1993-1-8, the design checks are:

The eccentricity moment from the gap between the supporting member face and the bolt group centroid is: M_Ed = V_Ed * a, where a is the distance from the bolt group centroid to the support face. For a typical fin plate with the bolt group 60 mm from the support face and a 10 mm gap: a = 60 + 10 = 70 mm. For V_Ed = 200 kN: M_Ed = 14.0 kN.m. This moment must be resisted by the bolt group.

Extended End Plate Connections — SCI P398 / SN041

Extended end plate moment connections transfer moment and shear from beam to column. The design method per EN 1993-1-8 Clause 6.2 uses the equivalent T-stub model:

T-Stub Model (Clause 6.2.4)

The tension zone of an end plate is modelled as a T-stub flange in bending. Three failure modes are possible:

Where:

• Mpl,1,Rd = 0.25 _ sum_leff,1 _ tf^2 * fy / gamma_M0
• Mpl,2,Rd = 0.25 _ sum_leff,2 _ tf^2 * fy / gamma_M0
• m = distance from bolt centre to plastic hinge at web/flange junction
• n = min(e_min, 1.25*m)
• e_min = edge distance from bolt centre to plate edge

For an M20 Grade 8.8 bolt in tension: Ft,Rd = 0.9 _ fub _ As / gamma*M2 = 0.9 * 800 _ 245 / 1.25 = 141.1 kN.

Prying Action

Prying forces amplify bolt tension beyond the applied load. EN 1993-1-8 accounts for prying through the T-stub model — Mode 1 and Mode 2 both include the effects of flange bending and prying. Mode 3 is pure bolt failure without prying. To minimise prying, use a stiff end plate (thickness typically 20-25 mm for M20 bolts).

Worked Example — Bolt Group in Combined Shear and Moment

Problem: Design a bolted bracket connection for a UKB 254x146x37 beam. The factored shear is V_Ed = 180 kN at an eccentricity of e = 120 mm from the bolt group centroid. Bolt group: 2 columns x 4 rows, M20 Grade 8.8, vertical pitch 70 mm, gauge 90 mm.

Step 1 — Design Moment at Bolt Group

M*Ed = V_Ed * e = 180 _ 0.120 = 21.6 kN.m

Step 2 — Bolt Group Properties

Vertical positions from centroid: y = {-105, -35, +35, +105} mm

Sum of y^2 per bolt column: 105^2 + 35^2 + 35^2 + 105^2 = 11,025 + 1,225 + 1,225 + 11,025 = 24,500 mm^2

Polar moment (both columns): sum*r^2 = 2 * 24,500 + 8 _ 45^2 = 49,000 + 16,200 = 65,200 mm^2

Step 3 — Shear per Bolt (Direct)

V_direct = V_Ed / n = 180 / 8 = 22.5 kN per bolt (vertical)

Step 4 — Shear per Bolt (Torsional)

Farthest bolt distance from centroid: r = sqrt(105^2 + 45^2) = sqrt(11,025 + 2,025) = sqrt(13,050) = 114.2 mm

Torsional force on farthest bolt: Vtorsion = M_Ed * r / sumr^2 = 21.6 * 10^6 * 114.2 / 65,200 = 37,830 N = 37.8 kN

Torsional force direction: perpendicular to radius vector. Angle of radius from horizontal: theta = atan(105/45) = 66.8 degrees. Torsional force angle: 66.8 + 90 = 156.8 degrees from horizontal (up and to the right), or 23.2 degrees from vertical.

Step 5 — Resultant Force on Farthest Bolt

Vertical components: V*direct (22.5 kN down) + V_torsion vertical component (37.8 * sin(23.2) = 37.8 _ 0.394 = 14.9 kN up for the two top bolts, down for the two bottom bolts)

For worst bolt (bottom row): V*vertical = 22.5 + 14.9 = 37.4 kN down V_horizontal = 37.8 * cos(23.2) = 37.8 _ 0.919 = 34.7 kN

Resultant: V_resultant = sqrt(37.4^2 + 34.7^2) = sqrt(1,399 + 1,204) = sqrt(2,603) = 51.0 kN

Step 6 — Capacity Check

Fv,Rd (M20 Grade 8.8, single shear, threads in): 94.1 kN

Utilisation: V_resultant / Fv,Rd = 51.0 / 94.1 = 0.542 — OK, 54% utilisation.

Bolt bearing on 10 mm S355 plate: Fb,Rd ~ 156 kN (from table above). Utilisation 51.0/156 = 0.327 — OK.

Result: 8-M20 Grade 8.8 bolts in 4x2 configuration is adequate. The connection is governed by bolt shear at 54% utilisation. A 4-bolt configuration (2x2) with larger lever arm or smaller eccentricity could be considered for optimisation.

Fillet Weld Design (EN 1993-1-8 Clause 4.5.3)

Two methods are permitted for fillet weld design:

Directional Method (Clause 4.5.3.2): The forces transmitted by a unit length of weld are resolved into components parallel and transverse to the weld axis, and normal to the throat plane:

[sigma_perp^2 + 3 * (tau_perp^2 + tau_para^2)]^0.5 <= fu / (beta_w * gamma_M2)
and
sigma_perp <= 0.9 * fu / gamma_M2

Simplified Method (Clause 4.5.3.3): The design resistance per unit length:

Fw,Rd = fvw,d * a
fvw,d = fu / (sqrt(3) * beta_w * gamma_M2)

Where:

• a = effective throat thickness (mm). For a 6 mm leg length fillet: a = 6 / sqrt(2) = 4.2 mm
• fu = nominal ultimate tensile strength of the weaker part joined
• beta_w = correlation factor (Table 4.1): 0.80 for S235, 0.85 for S275, 0.90 for S355, 1.00 for S420, 1.00 for S460
• gamma_M2 = 1.25

Fillet Weld Capacities — S355 (Simplified Method)

For a 6 mm fillet weld in S355 steel, fvw,d = 470 / (1.732 _ 0.90 _ 1.25) = 470 / 1.948 = 241.3 MPa. Per mm: Fw,Rd = 241.3 * 4.2 = 1,013 N/mm = 1.01 kN/mm. Per 100 mm: 101.3 kN.

The directional method yields approximately 15-20% more capacity for transverse welds compared to the simplified method, but requires checking both stress components.

Frequently Asked Questions

What is the difference between Category A, B, and C bolted connections in EN 1993-1-8?

Category A connections are bearing-type bolts (4.6 to 10.9 grade) loaded in shear where slip does not need to be prevented. They are the default for building structures. Category B connections use preloaded 8.8 or 10.9 bolts that are slip-resistant at the serviceability limit state — required where slip would impair structural performance. Category C connections are slip-resistant at the ultimate limit state — required for connections subjected to reversal, fatigue loading, or where holes are significantly oversize. Categories B and C require controlled tightening per EN 1090-2.

How is block tearing calculated in EN 1993-1-8?

Block tearing per EN 1993-1-8 Clause 3.10.2 is verified using two expressions and taking the minimum. Veff,1,Rd = fu _ Ant / gamma_M2 + (1/sqrt(3)) _ fy _ Anv / gamma_M0 and Veff,2,Rd = 0.5 _ fu _ Ant / gamma_M2 + (1/sqrt(3)) _ fy * Anv / gamma_M0. Ant is the net area in tension (perpendicular to the load), and Anv is the net area in shear (parallel to the load). The 0.5 factor in the second expression accounts for non-uniform stress distribution. The UK NA uses gamma_M2 = 1.25 and gamma_M0 = 1.00.

How does prying action affect bolt tension in end plate connections?

Prying action per EN 1993-1-8 Clause 6.2.4 amplifies bolt tension because end plate flexural deformation creates a lever effect that multiplies the applied tension at the bolt line. The T-stub model accounts for this through three failure modes: Mode 1 (complete flange yielding with large prying), Mode 2 (combined bolt failure and flange yielding with moderate prying), and Mode 3 (pure bolt failure without prying). Using thicker end plates (20-25 mm for M20 Grade 8.8 bolts) moves the failure mode from Mode 1 toward Mode 3, reducing prying amplification.

What is tying resistance and when is it required under Eurocode?

Tying resistance per EN 1993-1-8 Clause 6.2 and Annex A ensures structural integrity (robustness) so that in the event of accidental damage (e.g., gas explosion, vehicle impact), the structure does not suffer disproportionate collapse. EN 1991-1-7 classifies buildings into Consequence Classes (CC1, CC2a, CC2b, CC3). For CC2b and above (most multi-storey buildings), horizontal ties must resist a notional force of Ft = 0.8 _ (gk + psi _ qk) _ s _ L per tie, with a minimum of 75 kN in the UK. Tying resistance is checked separately from normal ULS design because the load factors and material factors differ (gamma_Mu = 1.10 for net section tie force per SCI P358).

How does the EN 1993-1-8 fillet weld design method compare to other codes?

The EN 1993-1-8 directional method is fundamentally similar to the AISC 360 instantaneous centre method but uses a stress-based interaction rather than a deformation-based approach. The beta*w correlation factor accounts for the reduced ductility of higher-strength steels (0.80 for S235 up to 1.00 for S460). For S355, the simplified method gives fvw,d = 241 MPa, compared to AISC phi = 0.75, which gives phi-Fnw = 0.75 * 0.60 _ 70 _ (1.0 + 0.5 _ sin^1.5 theta) for E70 electrodes — approximately 325 MPa for transverse loading. The EN approach is more conservative for transverse welds in lower-strength steels but comparable for longitudinal welds in S460.

Related Pages

• EN 1993 Steel Grades — S235, S275, S355, S460
• EN 1993-1-1 Beam Design — Flexure, Shear & LTB
• EN 1993 Column Buckling — Curves a0-d
• EN 1993 Moment Connections — Extended End Plate
• EN 1993 CHS & Hollow Section Joints
• Bolted Connections Calculator — Free EN 1993 Tool
• Beam Capacity Calculator — Free Online Tool

Educational reference only. Verify all design values against the current EN 1993-1-8 and the applicable National Annex for your jurisdiction. Connection design is project-specific — always check for the latest SCI/BCSA guidance. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent verification by a qualified structural engineer.